1. Field of the Invention
This invention relates generally to transducers having elements deformable in response to an applied electronic signal. In particular, this invention relates to an improved transducer of the bender type.
2. Description of the Prior Art
Bender transducers are well known in the prior art. For example, a piezoelectric bender for use as an acoustic transducer and mounted at the apex of a speaker cone is described in U.S. Pat. No. 3,548,116 issued to Hugo W. Schafft and assigned to the assignee of the present invention. A bimorph transducer element is described as being formed by two piezoelectric wafers which are fastened together by a metal shim which stiffens the bimorph structure and forms an electrode between the two wafers. A second U.S. Pat., No. 3,629,625, issued to the same inventor and assigned to the same assignee describes a center vane having corregations with the apices of the corregations cemented to the surfaces of the piezoelectric elements. The corregated portion of the center vane acts as a stiff hinge permitting movement of the piezoelectric elements relative to each other.
Piezoelectric elements for bender transducers in the prior art utilize thin, circular discs of piezoelectric material, each disc having flat opposing major surfaces with very thin electrodes deposited thereupon. The piezoelectric disc elements are polarized by applying an appropriate polarizing potential between opposite surfaces. Polarization of these elements aligns the electric dipoles in the material in a preferred direction. In operation, an electric potential is applied between the plates of a piezoelectric element which may either aid or oppose the polarization of the material. If the applied potential aids polarization the thin wafer will tend to increase slightly in the thickness dimension and to decrease slightly in the radial dimension. Conversely, if a voltage potential is applied which opposes the polarization of the piezoelectric element, the element will tend to slightly decrease its thickness dimension and to slightly increase its radial dimension. Thus, the thickness of a piezoelectric element can be controlled by an externally applied control potential.
When used in a single piezoelectric element bender, the piezoelectric disc is held in a fixed relationship with respect to a piezoelectrically inactive member. When the piezoelectric disc expands and contracts due to the voltage applied thereto, the piezoelectric disc and the piezoelectrically inactive disc bend and produce sound pressure waves in response to the applied signal voltage. Similarly, acoustic energy applied to the bender element will cause a resultant signal potential to be developed between the piezoelectric disc conductive faces.
In many prior art applications bilayer bender devices utilizing two piezoelectric wafers have been provided each wafer having thin conductive electrodes deposited on the faces thereof. A conductive center vane is provided for stiffly securing the piezoelectric wafer together and to provide an electrode between the inner faces of the piezoelectric wafers. The wafers are polarized and electrically connected such that one of the wafers expands radially outward while the other wafer contracts radially inward in response to a given potential being applied between the electrodes of each. This causes the assembly to have a bimorph action and to bend in an axial direction along the diameters of the wafers, causing the wafer to dish as one element expands and the other element contracts. Conversely, when acoustic energy is applied to a two layer bender, a piezoelectric voltage is generated.
A solid metal plate used as a center vane between two piezoelectric wafers, or between one piezoelectric wafer and a piezoelectrically inactive member provides good coupling between the elements in a radial direction but is too inflexible in the direction of bending, resulting in a lossy, inefficient structure. Corregated center vanes of the prior art have been stamped metal parts, requiring precise dimensional control and expensive tooling.
Prior art methods of supporting piezoelectric bender structures and diaphram cones have been limited to fastening the apex of the diaphram cone to the center of one of the piezoelectric disc elements. Since the mass of the piezoelectric bender structure is much greater than the mass of the diaphram cone and the load of the air mass driven by the diaphram cone, acoustic energy at higher frequencies will be radiated, or received, with higher efficiency at higher frequencies. This is because the momentum of the bender, being proportional to mass times the velocity, becomes less as the audio frequency decreases, the audio frequency determining the velocity.